Morphological changes with advancing age in rat cerebral cortex are to be investigated. The principal cell population to be studied consists of the pyramidal neurons of layers III and V in the primary visual cortex of albino rats. These cells are very close homologues of similar elements in human visual cortex. Animals in an age-graded sequence ranging from young adulthood (2 - 3 months old) to extreme old age (33 months old) will be studied. Three of the techniques to be used in this study are: Golgi impregnations, computer-assisted morphometry of dendrites, and deimpregnation of previously studied Golgi-impregnated neurons. Golgi-impregnated pyramidal neurons will be classified into discrete morphological subtypes, and these will be characterized light microscopically in quantitative fashion at all stages of the adult lifespan. Dendritic spines will be counted, using a new technique for estimating total spine population based on number of visible spines counted (Feldman and Peters, 1979). Dendritic and somal diameters will be measured. A computer linked to the microscope will be used to reconstruct true dendritic lengths and branching patterns in three dimensions. Golgi-impregnated cells will be gold-toned and deimpregnated by a technique which facilitates ultrastructural analysis of the same specific elements studied in the light microscope. In serial thin sections of deimpregnated reembedded neurons, cytoplasmic and synaptic changes with age affecting the cell body and dendrites will be described.